1. Field of the Invention
The present invention relates to computer data storage devices and, in particular, relates to a method of monitoring the temperature of an actuator coil of a hard disk drive by sampling the electrical properties of the coil.
2. Description of the Related Art
Hard disk drive storage devices are an important component in virtually all computer systems. In particular, hard disk drives provide computer systems with the ability to store and retrieve data in a non-volatile manner such that the data is maintained even if power is removed from the device. The popularity of these devices is based on their ability to quickly store and retrieve large quantities of digital information at low cost. However, because the computer industry continually strives to provide computer systems with increased performance, there exists a need for improved disk drives having increased data access speeds.
The typical hard disk drive comprises one or more pivotally mounted disks having a magnetic recording layer disposed thereon and a plurality of magnetic transducer elements for affecting and sensing the magnetization states of the recording layer. The recording layer comprises a large number of relatively small domains disposed thereon that can be independently magnetized according to a localized applied magnetic field and that can be maintained in the magnetized state when the external field is removed. The domains are grouped into concentric circular tracks each having a unique radius on the disk and data is written to or read from each track by positioning the transducer adjacent the disk at the corresponding radius while the disk is rotated at a fixed angular speed.
To position the transducer with respect to the disk, the typical hard disk drive further comprises a pivotally mounted actuator arm for supporting the transducer, a voice coil motor (VCM) for exerting a torque onto the actuator arm, and a servo-controller for controlling the VCM. The VCM comprises a coil of conducting wire wound into a plurality of loops and a permanent magnet disposed adjacent the coil. The servo-controller initiates movement of the actuator arm by directing a control current to flow through the coil which results in the permanent magnet applying a force onto the coil which is then transferred to the actuator arm in the form of a torque. Because the direction of the torque is dictated by the direction of control current flow, the servo-controller is able to reposition the transducer by first directing the control current through the coil so as to angularly accelerate the actuator arm in a first direction and then reversing the control current so as to angularly decelerate the actuator arm.
The time required to reposition the transducer in the foregoing manner is known as the xe2x80x9cseek timexe2x80x9d of the drive and is a critical performance factor that limits the throughput of the drive. For example, a drive having a short average seek time will generally be able to access a requested track of data more quickly than a drive having a longer average seek time. According to the state of the art, for a transducer having a linear acceleration greater than 500 meters/s2 or 50 g""s, the seek time required to reposition the transducer across a distance of 2.5 cm is typically in the range of 20-30 ms. Consequently, to provide such large acceleration, a relatively large (0.5 to 1A) current is often required to flow through the coil.
Unfortunately, when large amounts of current are directed through the coil, the rate of heat gain caused by the finite resistance of the windings of the coil may exceed the rate of heat loss to the environment. Thus, if left unchecked for an extended period of time, a rapid succession of seek operations may excessively raise the temperature of the coil such that the drive will no longer be operable. For example, when subjected to an instantaneous or average current that is beyond the VCM""s design limitations, the coil may generate excessive heat and rupture, or the coil overmold material may delaminate from the actuator assembly, lose its rigidity and/or outgas particulates into the disk drive enclosure, with deleterious results. Such outgassing from the coil overmold, coil insulators and/or from other materials applied to the coil wires (such as wire lubricants, for example) may occur even at relatively low temperatures (85xc2x0 C., for example). Thus, there is a need to monitor the temperature of the VCM coil so as to reduce the likelihood that the VCM will become damaged from such overheating.
One possible solution to the problem of excessive coil temperature is to blindly limit the VCM control current, i.e. without sensing or estimating the coil temperature, so as to be absolutely sure that the temperature of the coil is less than a threshold value. For example, following a first seek operation, a subsequent seek could be delayed so as to be sure that heat added to the coil during the first seek operation is substantially dissipated to the environment before the subsequent seek occurs. Alternatively, the resistive heat gain in the coil could be reduced by reducing the commanded current through the coil. However, because of the difficulty in estimating how well the environment can remove heat from the coil, the foregoing methods of blindly limiting the coil current will likely require overly conservative limitations. Thus, while possibly preventing the coil from overheating, the foregoing solution will likely result in unacceptably slow drive performance.
Another solution is proposed in U.S. Pat. No. 5,594,603 to Mori et al. and assigned to Fujitsu Limited, Japan. In this patent, the current applied to the coil is used to approximate the coil temperature. This method attempts to mathematically model the thermal behavior of the coil by inter-relating a group of factors that includes the VCM control current, the heat naturally radiated by the coil, the ambient temperature, and the thermal capacity of the coil. However, such modeling, although providing an indication of the present VCM temperature, may not accurately provide a calculated temperature value that reliably matches the present and actual temperature of the VCM.
Indeed, a number of factors may skew the results obtained from such mathematical models. For example, the present temperature of the drive or the resistance of the VCM coil may not remain constant and may thus result in changing VCM control current magnitudes. As the VCM control current is used as the basis for the temperature calculations, the VCM may not be driven (i.e., supplied with VCM control current) in an optimal manner and the actuator may not sweep as rapidly across the disk as it might otherwise have, thereby needlessly limiting the overall performance of the drive. Alternatively, should the mathematical model prove to be an inaccurate predictor of actual VCM temperature, in certain situations, excessive VCM control currents may be generated, potentially causing damage to the VCM and to the drive. Over many iterations, recursively-applied mathematical models may cause an initial and relatively small error to grow to such a degree that the model no longer accurately reflects present operating conditions. Reliance upon such an inexact mathematical model in modulating the VCM control current may understandably result in less than optimal drive performance characteristics.
Another proposed solution is proposed in U.S. Pat. No. 5,128,813 to Lee (hereafter the ""813 patent) and assigned to Quantum Corporation. In this patent, a discrete temperature-sensing element is used to dynamically sense the VCM temperature during the operation of the drive. This patent discloses that the thermistor is mounted for thermal conduction directly to the head and disk assembly. While the temperature sensing element may, in fact, provide a direct measurement of the temperature of the VCM (in contrast to the Mori et al. patent above, for example), this method requires mounting a high precision thermistor to the drive and requires that appropriate signal conditioning means be provided to measure, quantize and interpret the resistance of the thermistor. In many aspects, however, disk drive designers and manufacturers operate in all environment that has acquired many of the characteristics of a commodity market. In such a market, the addition of even a single, inexpensive part can directly and adversely affect competitiveness. In this case, therefore, the addition of the thermistor and associated signal conditioning means discussed in the ""813 patent maybe of little practical value.
From the foregoing, it will be appreciated that there is a need for improved methods of monitoring the temperature of a disk drive voice coil motor that are accurate, reliable and inexpensive in their implantation. Specifically, these methods should not rely upon complex and error prone mathematical modeling schemes or upon costly temperature sensing circuitry.
The aforementioned needs are satisfied by the hard disk drive according to one aspect of the present invention that stores information and receives current from a power supply. The hard disk drive comprises a magnetic medium having a plurality of magnetic domains disposed therein, wherein the magnetization states of the domains define the information stored on the hard disk drive. The drive further comprises a transducer for affecting and sensing the magnetization states of the magnetic domains and an actuator for moving the transducer between positions adjacent the magnetic domains. The actuator comprises an actuator coil that receives current from the power supply so that a conducting path is defined by the power supply and the actuator coil. The drive further comprises a control system for controlling the current flowing through the actuator coil, wherein the control system samples at least one electrical characteristic of the conducting path which is indicative of the temperature of the actuator coil so as to allow the control system to measure the temperature of the actuator coil.
In a second aspect, a method of regulating the temperature of an actuator coil of a hard disk drive comprises directing a current through the actuator coil, sampling at least one electrical characteristic of a conducting path defined by the actuator coil, extracting the temperature of the actuator coil from the sampled at least one electrical characteristic, and adjusting a current which is directed through the actuator coil according to the extracted temperature as to inhibit the temperature of the actuator coil from exceeding a threshold value.
In a third aspect, a method of regulating the temperature of an actuator coil of a hard disk drive comprises directing current through a conducting path defined by the actuator coil so as to accelerate a transducer of the hard disk drive, sampling at least one electrical characteristic of the conducting path, and adjusting the current which is directed through the actuator coil according to the sampled at least one electrical characteristic.
In a fourth aspect, a method of estimating the temperature of an actuator coil of a hard disk drive comprises developing a first parameter indicative of a resistive component of the voltage across the actuator coil, developing a second parameter indicative of a current flowing through the actuator coil, combining the first and second parameters to obtain an estimate of the resistance of the actuator coil, and extracting the temperature of the actuator coil from the estimated resistance of the actuator coil.
In a fifth aspect, a method of measuring the resistance of an actuator coil of a hard disk drive comprises directing current through the actuator coil, and sampling a voltage across the coil so as to obtain a sampled voltage value. The voltage across the coil includes a resistive component and a back emf component and the sampled voltage value includes a resistive component and a back emf component. The method further comprises sampling a current flowing through the coil so as to obtain an a sampled current value, multiplying the sampled current value by an adjustable factor so as to obtain an estimate of the resistive component of the sampled voltage value and subtracting the estimated resistive component of the sampled voltage value from the sampled voltage value so as to obtain an estimate of the back emf component of the sampled voltage value. The method further comprises successively adjusting the adjustable factor until the estimated back emf component of the sampled voltage value is approximately equal to the known back emf value, and extracting the resistance of the coil from the adjustable factor.
In a sixth aspect, a method of measuring the temperature of an actuator coil of a hard disk drive comprises directing a current through the coil, generating a first signal indicative of the voltage across the coil, generating a second signal indicative of the current flowing through the coil, and extracting the temperature of the coil from the first and second signals
In one embodiment, the method further comprises generating a third signal by incorporating an adjustable factor into the second signal, wherein the third signal is indicative of an estimate of a resistive component of the voltage across the coil. The method further comprises generating a fourth signal by combining the first and third signals, wherein the fourth signal is indicative of an estimate of a back emf component of the voltage across the coil. The method further comprises adjusting the adjustable factor until the estimated back emf component corresponding to the fourth signal is approximately equal to the expected back emf value.